Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 35,544	$ 65,569
Due after one year through five years	3,957	62,993
Due after five years through ten years	382,957	342,813
Due after ten years	368,708	446,552
Total debt securities available for sale	791,166	917,927
Estimated Fair Value
Due in one year or less	36,349	65,972
Due after one year through five years	3,994	64,656
Due after five years through ten years	399,180	354,238
Due after ten years	385,336	466,335
Estimated Fair Value	$ 824,859	$ 951,201